In certain types of wiper blades, the support element is meant to assure the most uniform possible distribution, over the entire field swept by the wiper blade, of the wiper blade contact pressure, originating in the wiper arm, against the window. By means of a suitable curvature of the unstressed support element—that is, when the wiper blade is not resting on the window—the ends of the wiper strip, which in wiper blade operation presses completely against the window, are stressed toward the window by the then-tensed support element, even if the radii of curvature of spherically curved vehicle windows change at every position of the wiper blade. The curvature of the wiper blade must accordingly be somewhat greater than that in the swept field, at the most pronounced measured curvature of the window to be wiped. The support element thus replaces the known, complicated support bracket construction with two spring rails disposed in the wiper strip and serving to reinforce the rubber-elastic wiper strip transversely (German Published, Nonexamined Patent Application DE-OS 15 05 257).
In wiper systems with a wiper blade that is guided over the window transversely to its length, a problem can arise which will now be explained in conjunction with FIG. 1 of the drawing: In the wiping motion that takes place in pendulum-type wiper system 10 (double arrow 12 between pendulum turning points 14), a wiper blade 18 pivotably connected to a driven wiper arm 16 cleans a wiping field 20 shaped like an annular segment, and the wiper arm 16 supported on one end swings like a pendulum, together with the wiper blade 18, about an axis 22. The wiper blade 18 is pivotably connected to the free end of the wiper arm 16 in such a way that the wiper blade can swing about an axis 24 oriented approximately parallel to the window and can adapt to the course of the window surface, which because of its shaping differs from the plane in which the wiper arm swings. The wiping field boundary is defined by the two pendulum turning points 14 and the wiper radii 26 and 28, of which the outer, longer radius 28 is defined by the end 30 of the wiper blade 18 remote from the pendulum axis, while the inner, shorter radius 26 is defined by the end 32 of the wiper blade 18 near the pendulum axis. A peculiarity of such wiper systems is considered to be that the speed at which the wiper blade 18 sweeps over the window increases from the inner radius 26 to the outer radius 28. Thus the forces of friction decrease steadily from the shorter radius 26 to the longer radius 28, as a function of the speed. The result is a moment acting on the known wiper blade that loads the wiper blade, or seeks to rotate it, about a vertical axis 34 perpendicular to the window. The alternating stress (double arrow 36) resulting as a function of the constantly changing pendulum direction (double arrow 12) acts on the support point between the wiper arm 16 and the wiper blade 18, which must therefore be designed as sufficiently stable. Vibrations caused by the moment in the wiper arm and in the wiper blade—which causes the wiper blade to exert a so-called “stick-slip” effect on the window—also worsen the outcome of wiping.
To overcome this deficiency, in a known wiper blade (German Published, Nonexamined Patent Application DE-OS 21 22 678), which for the sake of the most uniform possible distribution of the force originating in the wiper arm for pressing the wiper strip against the window is nevertheless provided with a multi-member support bracket frame, on whose end remote from the drive shaft, or from the wiper arm hub that can be connected to that shaft, of the support frame main bracket a leaf spring is disposed, which is fixed by its one end to the main bracket. On the other, free end of the leaf spring, which spring can be deflected in the direction of displacement of the wiper blade, a small lead plate is mounted on each side of the spring leaf. If during the wiping work the wiper blade begins to chatter—which essentially leads to vibration in the plane of motion of the wiper blade—then the leaf spring is also induced to vibrate. Because of the inertia of the small lead plates—given a suitable adaptation of the leaf spring in terms of its length and thickness and of the adapted magnitude of the weight of the small lead plates—the vibrations of the leaf spring are in the opposite direction, with a time lag, from the vibrations originating in the wiper blade and cancel the latter out.